A fuel cell is an electrochemical device that directly combines a fuel and an oxidant, such as hydrogen and oxygen, to produce electricity and exhaust, such as water. It has an anode or fuel electrode and a cathode or air electrode separated by an electrolyte. Hydrogen is oxidized to protons on the anode with an accompanying release of electrons. At the cathode, molecular oxygen reacts to form oxygen ions, consuming electrons in the process. Electrons flow from the anode to the cathode through an external load, and the circuit is completed by ionic current transport through the electrolyte. Hydrogen fuel cells do not emit toxic gasses. They operate quietly and have a potential efficiency of up to about 80 percent.
A particular class of fuel cells is thin film cells that include a solid electrolyte layer. This type of fuel cell is particularly well suited for applications such as small and microelectronics devices. Generally, the potential power output of a fuel cell varies nearly directly with the useful interface area between the electrolyte and anode and between the electrolyte and cathode. To provide increased output from thin film cells, longer and wider anode, cathode, and electrolyte layers have been provided. Enlarging these layers has required an enlarged footprint for the cells. This is undesirable in microelectronics devices.
Additional problems are known with solid oxide thin film fuel cells. For example, the cells typically require a relatively high operating temperature in the 700–1000° C. range. These high temperatures can lead to delamination problems in thin film cells. In particular, thermal cycling to and from these operating temperatures can cause substantial thermal expansion and contraction of the various fuel cell layers. Because the layers are made of different materials having different coefficients of thermal expansion, they may expand and contract by different amounts. This can cause the layers to come apart from one another.
Solutions to some of these problems have been proposed. For example, it has been proposed to construct cells using anode, cathode, and electrolyte layers having similar coefficients of thermal expansion. This can be difficult, however, since closely matching coefficients of thermal expansion do not necessarily correlate to increased cell performance.